and the spot can be detected only by the most careful investigation.
Not the least of the wonders of the eye are connected with the appreciation of images made upon the retina by certain parts of the brain. It is literally true that a person may see and yet not perceive. It has happened, in certain injuries of the brain, that a person sees and reads the words in a book and yet does not perceive their significance. This is called word-blindness. In a certain portion of the brain is a part which enables us to recognize the fact that we see an object; yet this object conveys no idea. There are two of these so-called centers of vision, one on either side, and their action is partly crossed. When the center is destroyed on one side, the inner half of one eye and the outer half of the other eye are blinded. Farther back in the brain, however, is a center which enables us to perceive or understand what is seen. When this center is destroyed we see objects and may avoid obstacles in walking, but persons, words, etc., are not recognized. This center exists only on the left side of the brain.
An impression, however short, made upon the retina is perceived. The letters on a printed page are distinctly seen when illuminated by an electric spark, the duration of which is only forty billionths of a second; but the impression remains much longer. Anything in motion appears to us in a way quite different from the single impression that we should have from an electric spark. In a picture representing an animal in motion, as it appears in an instantaneous photograph, the positions seems absurd and like nothing we have ever seen. In looking at a horse in action, the impressions made by the different position of the animal run into each other, and art should represent as nearly as possible the sum or average of these impressions. It is also true that impressions are diffused in the retina beyond the points upon which they are directly received. This is called irradiation; and the impression is diffused farther for white or light-colored than for black or dark objects. It is well known that a white square looks considerably larger than a dark square of exactly the same size; or the hands in white gloves look larger than in black gloves.
I have described, in as simple a way as possible, some wonderful things about the eye ascertained and explained by modern investigations; but there are many interesting facts ascertained which space has not permitted me to discuss, and there still remains much that is not yet understood. The whole question of the appreciation of colors and of color-blindness is still wrapped in mystery. We know that some persons can not distinguish between certain colors, but the reason of this is obscure. Perfect sight can exist only when the eye is perfect. The form and color of objects may be distorted so that an inaccurate